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Topic: EF 24-70 f/2.8L Replacement [CR2] (Read 35089 times)

So, bottom line, if you are waiting for a 24-70mm f/2.8L IS because you want a FF-equivalent of your 17-55mm IS, you can have that right now as the 24-105mm f/4L IS.

You're right, and that was exactly my point Jason was complaining that his 24-105/4L wasn't as good on his 5D2 as his 17-55/2.8 had been on his 50D, and I was surprised, because I thought the 24-105/4L would be similar but slightly better, if anything. I then went on and tested a 17-55/2.8 on a 7D and a 24-105/4L on a 5D2, and found that the latter was indeed very similar, but slightly better performer (for my copies).

But the bottom line is that I do not find slightly better compelling enough to go FF. A good 24-70mm f/2.8L IS on the other hand would convert me in an instant.

While the effective field of view on a crop camera may be the same as a longer lens on a 35mm, the focal length does not change and therefore neither does the aperture. A 17 - 55mm lens may give the same FOV as a 27 -88mm but it's still a 17 - 55mm and the relationship between the first element and the focal length remains unchanged as does the aperture.

You're right, but what is your point? The focal length just gives the image scale. 50mm/2.8 on FF gives exactly the same field (solid angle) as 80mm/4.5 on 1.6x APS-C (barring lens distortion). The flux of incoming photons per solid angle will be the same. The DOF will be the same. If the detectors have the same number of pixels with the same sensitivity (say fraction of photons actually detected, also called quantum efficiency) and read-out noise, dark current etc, then the resulting image will be statistically identical in all respects.

The lens is just an optical system to capture photons and compress them to a small image. The advantage of FF sensors is not that they are inherently more sensitive. The advantage is that it's easier to make optics that produce larger image scales (e.g., it's easier to produce a good EF 80mm/1.2 lens than an EF-S 50mm/0.75 lens).

Flake

[You're right, but what is your point? The focal length just gives the image scale. 50mm/2.8 on FF gives exactly the same field (solid angle) as 80mm/4.5 on 1.6x APS-C (barring lens distortion).

The point was in response to the previous post! aperture does not affect FOV, as you appear to be quoting however, and f/2.8 is important on Canon cameras because of the more sensitive focus centre spot.

Perhaps you would explain the change in focal length in the wrong direction? 50mm on FF is the same FOV as a 30mm (nearest equivalent) on a 1.6 crop, the aperture values would not change, DOF might depending on conditions

Perhaps you would explain the change in focal length in the wrong direction? 50mm on FF is the same FOV as a 30mm (nearest equivalent) on a 1.6 crop, the aperture values would not change, DOF might depending on conditions

I don't think I understand. Yes, 50mm on FF shows approximately the same field as 30mm on 1.6x APS-C. You didn't state the aperture values, so I don't know what you mean when you write that they don't change. In relation to what? Focal length and aperture (as in entrance pupil) are independent quantities, and the f-number is their ratio.

Let's put it this (trivial) way. 50mm/2.8 on 1.6x APS-C is exactly the same as 50mm/2.8 on FF, where only the central 1/1.6 = 62.5% (in linear dimension) of the frame is used.

With shorter focal length, you compress solid angle onto a smaller part of the detector; so with 31.25mm you compress the same field on the APS-C as the 50mm does on the FF. If we assume that the entrance pupil is the same, that is, the same number of photons per solid angle is captured for both lenses, then that implies that the photon density at the detector must be higher for the 30mm lens (because the same number of photons were compressed onto a 1.6^2 smaller area). That is, the f-number is sqrt(1.6^2) = 1.6 smaller.

I'm sorry if my explanations are confusing, I'm sure there is a good web page somewhere which explains things better.

Logged

Flake

Unfortunately that's not so epsiloneri, you're counting the crop factor twice.the final element of a 50mm f2.8 is 18mm on a 30mm 11mm (rounded out), but the ratio between these two is 1.6 (surprise!). If you then make allowance for the crop in a further calculation you count it twicw which makes the result wrong.

f/2.8 is a constant regardless of the focal length if the lens is pointed at a uniform light source the light reaching the sensor should be constant across its surface. Therefore the size of the sensor is unimiportant, as is the crop factor. A slightly more meaningful figure would be to use the pixel density, but even then as sensors peform differently it's difficult to make the test accurate enough to make any sense.

A FF camera and a crop camera pointed at the uniform light source with say a 50mm & 30mm both at f/2.8 and the same ISO should return the same shutter speed for correct exposure.

Just to make it more complicated though, not all cameras share the same bayer matrix, they can be stronger or weaker, then there's the anti aliasing filter, and they're not all the same either, not the microlenses. It's very difficult to compare like with like.

Just to make it more complicated though, not all cameras share the same bayer matrix, they can be stronger or weaker, then there's the anti aliasing filter, and they're not all the same either, not the microlenses. It's very difficult to compare like with like.

Yes, there are other effects, but they are less significant by comparison ("second order corrections") and more about engineering than fundamental physics.

So you don't agree that 50mm/2.8 on a FF is identical to 31.25mm/1.75 on 1.6 APS-C? Or is it something else?

Yes I don't agree with that, f/2.8 is f/2.8 regardless of the crop factor

Ok, good, then I understand. You are right, that for the same lens the aperture doesn't change when you go from FF to crop. That would be weird. But now think of two different lenses that both share the same entrance pupil - say 18mm in both cases - but where the focal length is different, namely 50mm in one case and 31.25mm in the other. Then the image on a FF with the 50mm lens would be identical to the image of the 31.25mm on the 1.6x APS-C. And, also note, since the entrance pupil is the same, the f/#-number changes: 50/18 = 2.8 while 30/18 = 1.75. Which is what I stated in the quote above.

While the effective field of view on a crop camera may be the same as a longer lens on a 35mm, the focal length does not change and therefore neither does the aperture. A 17 - 55mm lens may give the same FOV as a 27 -88mm but it's still a 17 - 55mm and the relationship between the first element and the focal length remains unchanged as does the aperture.

True. But not really the point I am making... 55mm on a crop body is equivalent to 88mm on FF, and f/2.8 is f/2.8 regardless. But for the same framing, you'd be closer to the subject with a FF body, meaning shallower DoF. It also means the perspective is different, so it's not really the same shot. But from a technical standpoint, on FF DoF is effectively shallower, and when comparing current FF sensors to current crop sensors, ISO performance is improved so one can increase ISO by a stop to 'make up the difference' between f/2.8 and f/4 (in terms of shutter speed) with no noise penalty.

What that means, and my main point, is that functionally the 24-105mm f/4 on FF is approximately equivalent to the 17-55mm f/2.8 on a crop body (from a DoF and exposure standpoint, plus it has a broader focal range). So the "I'm not going FF yet because there's no FF equivalent to the 17-55mm f/2.8 IS" argument is a fallacy, since on FF, f/4 + better ISO performance is comparable to 1.6x f/2.8, and both lenses have IS.

Now, if someone is waiting for f/2.8 IS on FF because the whole point of moving to FF is to achieve better performance, rather than comparable performance (from a technical standpoint), I can't argue with that - so, if that's the case, keep on waiting for the 24-70mm f/2.8L IS.

Flake

One of the problems is that f/2.8 is a magic number for Canon cameras and gives a brighter viewfinder, and more accurate autofocus.The current f/2.8 24 -70mm is a far better lens than the 24 - 105mm f/4 IS and I'm speaking from experience owning both of them, just sold the f/2.8 in anticipation of a new one before values fall.Photozones test of the 24 - 105mm f/4 IS on FF wasn't what I wanted to see, it has problems with edge softness vignetting, distortion, and its minimum focus distance and magnification leave a lot to be desired. It's useless for close up work, the 17 - 55mm is much closer to the 24 - 70mm here.The saving grace of the 24 - 105mm is the IS which the 24 - 70mm doesn't have, when it finally gets it I think the 24 - 105mm will be going on Ebay!

So you don't agree that 50mm/2.8 on a FF is identical to 31.25mm/1.75 on 1.6 APS-C? Or is it something else?

Yes I don't agree with that, f/2.8 is f/2.8 regardless of the crop factor

Ok, good, then I understand. You are right, that for the same lens the aperture doesn't change when you go from FF to crop. That would be weird. But now think of two different lenses that both share the same entrance pupil - say 18mm in both cases - but where the focal length is different, namely 50mm in one case and 31.25mm in the other.

Yes, f/2.8 is f/2.8 and the light per unit area of the sensor is the same, independent of sensor size. Likewise, the focal length of a lens doesn't change on crop vs. FF, just the angle of view (i.e. the magnification is the same). The effect on DoF is an apparent one, seen in practice, even with the same lens. If I frame a head shot with an 85mm f/1.2 @ f/1.2 on my 7D, then I say 'whoops, what am I thinking' and move the 85/1.2 onto a 5DII without moving my feet or the subject, instead of a nice, tight head shot I'll have a lot of background around the subject. So, I'll take a few steps forward to get that nice tight framing again, and that decreased distance-to-subject translates to a shallower DoF - by a factor of 1.6x.

Photozones test of the 24 - 105mm f/4 IS on FF wasn't what I wanted to see, it has problems with edge softness vignetting, distortion, and its minimum focus distance and magnification leave a lot to be desired. It's useless for close up work, the 17 - 55mm is much closer to the 24 - 70mm here.

As I already wrote, according to the photozone tests themselves, 24-105mm/4.0L on 5D2 is at least as good as 17-55mm/2.8 on 7D, at least if we're talking IQ (7D has other advantages). I confirmed it with my own tests. In other words, the 17-55mm/2.8 on 7D is just as useless as 24-105mm/4.0L on 5D2.

f/2.8 on APS-C gives about the same viewfinder brightness as f/4.5 on FF. Why is that? Because in order to view the full field of view in the same solid angle, the FF viewfinder needs to compress the image (almost) 1.6x more than the APS-C viewfinder -> increasing the surface brightness. This is easy to test yourself: mount the 17-55/f2.8 on a 7D and the 24-105/4L on a 5D2. Which viewfinder is brightest? They're about the same (the 5D2 has a slightly larger apparent field in the viewfinder, but it's compensated by the lens being 4.0 instead of 4.5). If you don't believe me try it! (Of course, your 24-70mm/2.8L lens will be much brighter on the 5D2.)

So, I'll take a few steps forward to get that nice tight framing again, and that decreased distance-to-subject translates to a shallower DoF - by a factor of 1.6x.

Yes, but the perspective will have all changed, so it's not equivalent. I think we both agree that, in contrast, the imaged formed by a 50mm/2.8 lens on a FF sensor is actually equivalent to the image formed by a 31.25mm/1.75 lens on a 1.6x APS-C sensor. Actually equivalent (identical!), in terms field of view, photon collection rate per solid angle, depth of field. No difference.